When employers who self-fund their employee benefits form a larger group in order to offer health insurance benefits to each employer's workers, it is called a (an) :

Multiple Employer Welfare Association

Explanation:

A multiple employer welfare association (MEWA) is formed by at least 10 employers who want to lower the tax implications of providing life insurance benefits to their employees.

The whole group combines their contributions into a self-contributing health insurance plan that benefits their employees.

MEWAs are usually formed by small companies, since large corporations with tens of thousands of employees can do the same but on their own without needing other companies.